A lot of reasons! Some I have heard:

- Homeownership comes with responsibilities that people may not want

- They prefer to invest the money elsewhere

- They expect to relocate on timeframes shorter than those that make homeownership a good investment

I'm asking, what causes people to buy a house for the first time?

For upper middle class people the answer is probably having a baby, so you can see why if people are having babies later in life when their incomes are higher, prices rise.

but the upper middle class is relatively small, depending on how you define upper middle class. Nationally, one might say the 15% who are below the 95th percentile (but locally is what matters for the purpose of buying property, and maybe "upper middle" is 60% to 95% of the local ranking).